It is a well known fact that processes based on the use of carbon dioxide occupy a preferred place among the secondary and tertiary methods of mineral oil exploitation, particularly in Hungary. For several reasons these inexpensive and promising methods have not yet reached the level of efficiency which was expected from them. A very important reason is one to which heretofore neither researchers nor designers paid sufficient attention. A special branch of clay mineral colloid chemistry deals with ion exchange processes that take place in clay minerals, tending to swell and having a layer structure of 2:1. If in these clay structures Ca and Mg cations are replaced by monovalent cations, contraction and swelling phenomena take place which considerably change outer and inner surface properties, sorption and stability.
These scientific observations were recognized and accepted by producing branches working under relatively static rock conditions, such as in the case of deep-drilling. However, no active correlation was established between reservoir-mechanism based on the occurrence of hydrodynamic processes in porous systems under dynamic circumstances, and the connected physical and chemical observations. For this reason the damaging role of clay minerals, in particular of clay minerals capable of swelling in the storing rocks, has not been sufficiently examined, although these phenoemena occur nearly in all cases in which liquid is injected and especially when water, aqueous solutions, liquids with acidic character or liquids tending to losing protons are injected. One of the reasons for the low efficiency of these processes when using carbon dioxide is in that clay minerals in the storing rocks become unbalanced during injection. This exerts a considerable effect on the hydrodynamic properties, sorption and stability of the storing rocks.
When in secondary and tertiary oil exploitation carbon dioxide is injected into the strata, in addition to multiphase liquid flow also interactions arise between rock and the liquid which are damaging the rocks. The rock-dissolving effect of carbon dioxide is mentioned, but no attention was paid to the damage caused by the clay minerals due to built-in H.sup.+ ions.
It is also well known that clay minerals do not only perform rock-formation but they are cementing the rocks as natural consolidating materials. That means that built-in H.sup.+ -ions thicken the hydrate layer. This leads to swelling and dispersion, not only of clay minerals but of the entire storing rock cemented by the clay minerals is dispersed, resulting in phenomena such as clogging of production wells with sand, and continuous injectivity loss of injection wells.
In addition to the effect of the injected carbon dioxide causing dissolution of carbonates, also rock dispersion of significant extent occurs. During carbonate dissolution detrimental cations such as Na.sup.+ get into the aqueous phase returned into the clay structure and cause considerable swelling and dispersion. Since carbon dioxide injection lasts for a longer period, and this is followed by injection of water, and since in these waters calcium and magnesium are substituted by sodium, it becomes clear that a continuous dispersing process takes place. Since this helps the natural system, considerable channel formation takes place, injected water and later the aqueous phase is running quickly through the storing rock and as a result flooding efficiency will be low.